1.9.2Physical Chemistry

EMF of Electrochemical Cells — Oxford AQA International A Level Chemistry Revision Notes & Topical Questions

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📚 Oxford AQA International A Level Chemistry Hub / Topical Revision

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Part 1 of 6Calculating EMF

The EMF is the maximum potential difference of the cell under standard conditions.

Calculating EMF

Formula

E⦵cell = E⦵(positive electrode) − E⦵(negative electrode), where both values are taken from the table of reduction potentials.

Positive electrode

The half-cell with the more positive (or less negative) E⦵ gains electrons and is reduced.

Negative electrode

The half-cell with the more negative E⦵ loses electrons and is oxidised, so its half-equation is reversed.

No scaling

E⦵ does not change when a half-equation is multiplied, because potential is independent of the amount of substance.

Part 2 of 6Predicting Reactions

List the two half-equations as reductions with their E⦵ values and identify which is more positive.

The more positive half-equation runs forwards, and the other runs backwards, giving the overall reaction.

Balance the electrons, add the half-equations and cancel the electrons to obtain the full equation.

Calculate E⦵cell; a positive value means the reaction is feasible under standard conditions.

Remember a feasible reaction may still be extremely slow if its activation energy is high.

Part 3 of 6Practical: Measuring EMF

Preparation

Clean each metal strip with sandpaper and degrease it with propanone so that oxide layers do not affect the reading.

Half-cells

Place each metal in a beaker containing 1.00 mol dm⁻³ solution of its own ions, using equal volumes.

Salt bridge

Connect the beakers with a strip of filter paper or tube soaked in an unreactive electrolyte, preparing it fresh for each cell.

Reading

Connect a high-resistance voltmeter with crocodile clips and record the stable value, noting which terminal is positive.

Comparing

Repeat with different pairs of metals and compare with calculated EMF values, which should agree closely if conditions are standard.

Part 4 of 6Limitations of Predictions

Limitations of Predictions

Conditions

Real concentrations, temperatures and pressures differ from standard values, which shifts the equilibrium positions and the EMF.

Rate

E⦵ tells us whether a reaction is feasible, not how fast it is, as kinetic barriers can prevent change.

Measurement

A voltmeter with insufficient resistance draws current, so the measured value falls below the true EMF.

Half-equation	E⦵ / V
Zn²⁺ + 2e⁻ ⇌ Zn	−0.76
Cu²⁺ + 2e⁻ ⇌ Cu	+0.34
Ag⁺ + e⁻ ⇌ Ag	+0.80
Fe³⁺ + e⁻ ⇌ Fe²⁺	+0.77
I₂ + 2e⁻ ⇌ 2I⁻	+0.54

Part 5 of 6Worked Example

Question Use the table to find E⦵cell for Fe³⁺/Fe²⁺ with I₂/I⁻ and write the feasible reaction.

Fe³⁺/Fe²⁺ (+0.77 V) is more positive than I₂/I⁻ (+0.54 V), so Fe³⁺ is reduced and I⁻ is oxidised.
E⦵cell = 0.77 − 0.54 = +0.23 V.
Balance electrons
12Fe³⁺ + 2e⁻22Fe²⁺ and 2I⁻3I₂ + 2e⁻
Add2Fe³⁺(aq) + 2I⁻(aq) → 2Fe²⁺(aq) + I₂(aq).

Answer E⦵cell = +0.23 V; reaction is feasible

Part 6 of 6Exam Checklist & Tips

1.9.2 – Key Points for the Exam

EMF The maximum potential difference between the two electrodes of a cell, measured with no current flowing.
Reduction potential The potential for a half-equation written with electrons on the left.
Feasible reaction One with a positive E⦵cell under standard conditions.
Oxidising agent A species that accepts electrons and is reduced.

Common Mistakes

Doubling E⦵ when a half-equation is multiplied by two.
Subtracting the positive electrode from the negative one, giving the wrong sign.
Saying a positive EMF means a fast reaction.
Forgetting to reverse the half-equation with the lower E⦵ when writing the full equation.

Last updated: 28 October 2026

Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A ⇔ A = I / M ⇔ I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.